Vue（八）：监听属性watch

 Vue.js 可以通过 watch 来响应数据的变化。

以下实例模拟购物车里商品数量增加，对应价格也增加

       <!--模拟购物车商品数量增加，价格也随之增加-->
       <div id = "app">
            
         <h3>Search Goods</h3>
         <input type='goodsname' v-model="goodsname" />
         <br><br>
         <table border=1>
            <tr>
                <th>goodsname</th>
                <th>spnums</th>
            </tr>
            <tr>
                <td>{{goodsname}}</td>
                <td>{{spnums}}</td>
            </tr>
         </table>
         
         <hr>
            
         <p style="color:red">{{msg}}</p>

         <h3>Shopping Cart</h3>
         <p>apple: {{ num }} </p>
         <p>price: {{ price }}</p>
         <button @click = "num++" >add apple num</button>
        </div>
        <script type = "text/javascript">
         var randomnum = Math.floor(Math.random()*100);
         var vm = new Vue({
            el: '#app',
            data: {
               num: randomnum,
               price:156,
               msg:'',
               goodsname:'',
               spnums:randomnum
            },
            watch:{
                goodsname:function(val){
                    if(val=='a' || val=='ap' || val=='app' || val=='appl'
                       || val=='apple'){
                        this.goodsname='apple';
                    }else{
                        alert('no result!');
                    }
                }
            }
         });
　　　　　　// $watch是一个实例方法
         vm.$watch('num', function(nval, oval) {
             this.spnums--;
             this.msg="cart apple num add,from "+oval+" to "+nval+",so price change to "+(this.price+=10)
         });
        </script>